Collapse of genetic division of labour and evolution of autonomy in pellicle biofilms

Abstract

Closely related microbes often cooperate, but the prevalence and stability of cooperation between different genotypes remains debatable. Here, we track the evolution of pellicle biofilms formed through genetic division of labour and ask whether partially deficient partners can evolve autonomy.Pellicles of Bacillus subtilis rely on an extracellular matrix composed of exopolysaccharide (EPS) and the fibre protein TasA. In monocultures, ∆eps and ∆tasA mutants fail to form pellicles, but, facilitated by cooperation, they succeed in co-culture. Interestingly, cooperation collapses on an evolutionary timescale, and ∆tasA gradually outcompetes its partner ∆eps. Pellicle formation can evolve independently from division of labour in ∆eps and ∆tasA monocultures, by selection acting on the residual matrix component, TasA or EPS respectively. Using a set of interdisciplinary tools, we unravel that the TasA-producer (∆eps) evolves via an unconventional but reproducible substitution in TasA that modulates the biochemical properties of the protein. On the contrary, the EPS-producer (ΔtasA) undergoes genetically variable adaptations, all leading to enhanced EPS secretion and biofilms with different biomechanical properties. Finally, we revisit the collapse of division of labour between Δeps and ΔtasA in light of a strong frequency vs. exploitability trade-off that manifested in the solitarily evolving partners. We propose such trade-off differences may represent an additional barrier to evolution of division of labour between genetically distinct microbes.

Biofilms 2018: a Diversity of Microbes and Mechanisms

“…Ákos T. Kovács (Technical University of Denmark, Kgs Lyngby, Denmark) described experiments in which mixtures of mutants lacking one or the other component could successfully form biofilms and improve productivity when the strains were present at the right ratios (30% protein-producing strains and 70% EPS-producing strains) (88). In subsequent evolution experiments, the tasA mutants began to dominate in the mixed biofilm, resulting in altered biofilm structure (89). The evolved eps mutant acquired mutations in tasA, resulting in the introduction of cysteine residues that improved pellicle strength, while evolved tasA mutants acquired mutations that increased eps production.…”

Cheaters shape the evolution of phenotypic heterogeneity in Bacillus subtilis biofilms

“…In addition, our work brings up two major findings 1) matrix producers can adapt to matrix nonproducers by shifting phenotypic heterogeneity towards increased number of matrixoverexpressing cells 2) hyper ON phenotype is associated with favorable positioning of the matrix producers in the biofilm in presence of cheats, thereby limiting their numbers 3) hyper ON anticheating strategy is a short-term solution, which can either be replaced by another, yet unknown 15 strategy linked to hyper OFF phenotype, or followed by tragedy of the commons. As recently demonstrated, alternative EPS-independent biofilm formation strategies can emerge by single amino acid change is TasA [45]. It remains to be discovered whether shifts in phenotypic heterogeneity in response to long term cheating is general phenomenon that applies to different types of public goods.…”

“…To address this question, we co-cultured the EPS producers (wild type-WT) and cheating strain (∆eps) for 10 biofilm growth cycles (~60 generations) (see methods). Based on previous studies [44,45], we assumed that this evolutionary timeframe will be sufficient for evolution of adaptive mechanisms in the WT in response to social cheating, at the same time preventing the 9 diversification of the WT into a biofilm-deficient morphotype, which can be observed on longer evolutionary timescales [44].…”

Endosymbiosis before eukaryotes: mitochondrial establishment in protoeukaryotes

“…If secretion rates are increased, it renders the collective more vulnerable to cheaters [133]. In in vivo experiments an initially complementary cooperation can easily collapse as one of the products is overdosed, while the other lags [135]. This leads to the vanishing of one product and either one or both species (depending on how dependent they were on each other).…”